Rising jet fuel costs could produce losses totalling as much as $4bn (£2.2bn) for airlines this year, the International Air Transport Association has warned.

It calculated that extra fuel costs could add as much as $10bn to the global airline industry's bills and wipe out any gains from improving passenger numbers.

The airline industry has remained fragile since 11 September 2001, with many of the US' and Europe's biggest carriers struggling financially.

Nigeria crisis

The mounting unrest in Nigeria's Delta region is the latest of many factors troubling world oil markets, from instability in the Middle East, to hurricanes in the Caribbean and the pressure of stronger economic growth on global demand.

Shell and Agip have both evacuated non-essential workers from the south Nigerian Delta in recent days where the government is fighting insurgents.

"The problems of Nigeria have obviously got worse," said Robert Laughlin, a trader at GNI-Man Financial. "This is a major problem that we don't need because they produce sweet crude oil. There is not enough sweet crude oil."

This high quality, low sulphur crude oil is commonly used for processing into gasoline and is in high demand, particularly in the US and China.

Rebel fighters have threatened "all out war" in the country's Niger Delta region but oil firms remain confident that the situation is far from being that grave.

A Shell spokesman in Lagos said the company had received "threats like this in the past", and described the staff evacuations as "precautionary".